More robust listeria risk assessment by including heat-injured cells

Food safety protocols and standards are well established, but Listeria contamination has been recently traced to niches in the food processing environment that harbor the bacteria, according to dairy science professor Sanjeev Anand. For instance, Listeria contamination in one commercial ice cream plant was traced to bacteria on the spout of an ice cream freezer.

Each year, an estimated 1,600 Americans become ill due to eating foods contaminated with Listeria -- it's the third-leading cause of death from foodborne illnesses, according to the Centers for Disease Control and Prevention. Listeria poses the greatest danger to children, pregnant women, the elderly and those with compromised immune systems.

"Listeria is a cold-loving microorganism," explained Anand. Pasteurization and cooking kills listeria, but the bacteria can grow at temperatures 40 degrees Fahrenheit and above in refrigerators and can even survive freezing.

To help the dairy processing industry combat Listeria, Anand and doctoral student Neha Neha are developing risk assessment models to more accurately predict the risk from Listeria, especially due to the recovery potential of any injured cells. These models use product matrix parameters and environmental ones, such as storage temperature, duration, pH and water activity along with the potential levels of cross contamination from the environment.

They are working with associate professor Gemechis Djira of the Department of Mathematics and Statistics to create regression models. The research is supported by the Midwest Dairy Foods Research Center and the South Dakota Agricultural Experiment Station.

Although the injured cells are not known to cause illness, they may have the ability to recover and repair themselves, Neha explained. She presented her findings at the American Dairy Science Association annual meeting in Pittsburgh last summer and placed third in the 3-minute Graduate Student Thesis Contest.

To better understand the risk from injured cells, Neha looked at the organism's behavior in different ice cream mixes with total solid levels ranging from 36 to 45 percent. She spiked them with three levels of a nonpathogenic Listeria strain before pasteurization. Results showed that injured listeria cells did not recover in the ice cream mix itself under the normal conditions of mix handling.

For her work on Listeria, Neha was awarded the 2018 Joseph F. Nelson graduate scholarship, which recognizes outstanding scientific research at South Dakota State.

To address the issue of cross contamination in the manufacturing environment, the researchers must determine how Listeria builds up in the environment, what characteristics make this possible and how it resists clean up.

"On the science side, we want to understand what characteristics make it possible for Listeria to recover," Neha said. To examine persistence, she will do whole genome sequencing of the bacteria, with the goal of understanding the gene expression that leads to colonization. It will also help her compare any resident strain of Listeria, which can form resilient biofilms in the harborage sites and are difficult to eradicate.

"If we gain more knowledge about injured cells and integrate that among the variables for the product and the environmental site, this can help us design more robust risk-assessment models," she explained. In addition, examining the organisms at a molecular level will help scientists design new cleaning techniques that can eliminate the bacteria in the manufacturing environment and prevent their persistence.

Anand emphasized that this type of research can help dairy and food-manufacturing companies worldwide enhance food safety protocols and thus protect the public from foodborne illnesses.